US Secretary of State Rex Tillerson makes his diplomatic debut at a G20 gathering in Germany Thursday, where his counterparts hope to find out what "America First" means for the rest of the world.

Host nation Germany has billed the two-day meeting as a chance for the club of leading economies to discuss how to work together on challenges ranging from climate change to the conflicts in Syria, Ukraine and Yemen.

But all eyes will be on America's top diplomat, whose maiden trip to Europe will bring him face-to-face with allies seeking reassurance that President Donald Trump will not upend decades of US foreign policy despite his fiery campaign rhetoric.

Tillerson will also have his first sitdown with Russian Foreign Minister Sergei Lavrov in Bonn, talks that will be closely scrutinised for any hint of a rapprochement as controversy swirls over White House ties to the Kremlin.

The Texan could also face questions about the Israeli-Palestinian conflict, after Trump caused international consternation Wednesday by dropping Washington's years-long quest for a two-state solution, saying he would back a single state if it led to peace.

US relations with China are likewise in the spotlight. The G20 gathering could see Tillerson's first encounter with Chinese Foreign Minister Wang Yi, whose attendance was only confirmed days earlier following a conciliatory phone call between Trump and President Xi Jinping.

China "is working out the schedule on bilateral meetings," ministry spokesman Geng Shuang said, adding that Trump and Xi Jinping had had a "very good" conversation.

During the call, Trump reaffirmed US adherence to the decades-old position that Taiwan is not separate from China, backtracking on earlier comments that cast doubt on the "One China" principle.

If former ExxonMobil boss Tillerson faces the daunting task of clarifying Trump's, at times, contradictory policy signals to a wary international community, he will not be doing it alone.

US Defence Secretary James Mattis is attending the second day of a NATO meeting in Brussels Thursday, while Vice President Mike Pence will be the highest-ranking US official participating in the Munich Security Conference from Friday.

"There are plenty of uncertainties about what they want, what they plan," a European diplomat said about the Trump administration. "We hope we'll get some clarity in the weeks to come."

- 'Your children's security' -

Speaking in Brussels on Wednesday, Mattis reaffirmed Washington's commitment to NATO after Trump branded the Western military alliance "obsolete".

But the former Marine also sternly rebuked member states for not paying their fair share, and warned that Washington would "moderate its commitment" to the alliance unless they boost their spending.

"Americans cannot care more for your children's future security than you do," he said.

Looming large over this week's US diplomatic offensive is the scandal surrounding the resignation of Trump's national security advisor Michael Flynn.

He was forced to step down after less than a month in the job on allegations he discussed US sanctions against Moscow over the Ukraine conflict with a Russian official before taking office.

In Bonn, Tillerson will likely also be grilled about Trump's commitment to the European Union after he praised Britain's decision to quit the bloc and predicted other countries would follow suit.

By contrast, UN chief Antonio Guterres, another Bonn attendee, said in an interview with British newspaper The Guardian that a "united Europe" had a crucial role to play at a time of global upheaval.

He urged the bloc to "learn the lessons of Brexit".

The G20 event is hosted by German Foreign Minister Sigmar Gabriel, who has been a vocal critic of Trump's protectionist and nationalist leanings.

"No country in the world can tackle the major international problems of our time alone," Gabriel said ahead of Thursday's talks.

The G20 grouping accounts for some 85 percent of the global economy and two thirds of the population.

Other high-profile diplomats attending include Turkey's Mevlut Cavusoglu, the EU's Federica Mogherini, and Britain's Boris Johnson.

The meeting is a precursor to a G20 leaders' summit scheduled to take place in Hamburg in July, likely to be the first time Trump will meet Russian President Vladimir Putin in person.

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